The need for treating sewage sludge for disposal, and particularly municipal sewage sludge, has increased substantially in recent years. One way of doing so, is by mixing or blending alkaline materials with the sludge. There is also developing an increasing demand, particularly with respect to municipal sewage sludge, that the treatment of the sludge be sufficiently thorough that is will allow beneficial re-use of sludge, such as enabling the sludge to be applied to land on which agricultural and ornamental crops may be grown. In an effort to make stabilized sludge more marketable, those treating the sludge have attempted to produce a granular, scatterable product having a soil-like texture. One way of doing so is to mix alkaline products with sludge in a manner that blends the alkaline and sludge and transforms the otherwise pasty sludge into an acceptable end product.
However, conveying and mixing wastewater sludges are difficult functions due to certain inherent characteristics of wastewater sludges. Some such characteristics are the thixotrophic nature of the sludges, the variations in the sludge characteristics depending upon the process or methods that may be used in the wastewater process, the type of sludge that is produced as a function of the type of community, variations in amounts of solids present in the sludge, and variations in the properties and characteristics of the alkaline materials that are used for stabilization of sludge, in those instances where alkaline materials are used.
Prior to the invention hereof, other methods of mixing and/or blending sludge have been known, including the methods of Wurtz, U.S. Pat. No. 3,941,357 and U.S. Pat. No. 4,306,978; Manchak, U.S. Pat. No. 4,079,003; Roediger, U.S. Pat. No. 4,270,279 and Nicholson, U.S. Pat. No. 4,554,002 for example.
Wurtz U.S. Pat. No. 4,306,978 teaches mixing the sludge by using a series of plowshare working tools operating at rapid speeds to mechanically divide and separate particles, exposing the surfaces before coating them with lime.
Manchak U.S. Pat. No. 4,079,003 teaches the use of longitudinally spaced paddles for moving and agitating a mixture, particularly when the minimum solids concentration is in excess of 25%, so as to achieve an agglomeration of solids into a solid friable material, largely as a result of large quantities of lime to generate heat and cause dehydration and solidification.
Roediger U.S. Pat. No. 4,270,279 teaches combining sludge and alkaline materials without intimate mixing, but by dusting the surface only, as his invention, aimed at breaking up the sludge for sprinkling with quick lime in a simple paddle mixer.
Nicholson U.S. Pat. No. 4,554,002 teaches the necessity of a curing step, of, for example, three days duration.